1. Field of the Invention
The present invention relates to a light emitting device formed of a material having a piezoelectric effect.
2. Description of the Background Art
Semiconductor light emitting devices such as semiconductor laser devices and light emitting diodes using III group nitride semiconductors such as GaN, GaInN, AlGaN, and AlGaInN (hereinafter referred to as nitride based semiconductors) have been expected to be applied as light emitting devices for emitting light in a visible-ultraviolet region.
Out of the applications, extensive development has been proceeding toward practical applications of the semiconductor light emitting devices having a GaInN quantum well layer as a light emitting layer. Such semiconductor light emitting devices have been fabricated on a (0001) surface of a substrate composed of sapphire or silicon carbide (SiC) by MOVPE (Metal Organic Vapor Phase Epitaxy) or MBE (Molecular Beam Epitaxy).
FIG. 42 is a schematic sectional view showing the structure of a conventional GaN based semiconductor light emitting device. The semiconductor light emitting device shown in FIG. 42 is disclosed in JP-A-6-268257.
In FIG. 42, a buffer layer 62 composed of GaN, an n-type contact layer 63 composed of n-GaN, a light emitting layer 64 having a multiple quantum well (MQW) structure, and a p-type cap layer 65 composed of p-GaN are formed in this order on a sapphire substrate 61. The light emitting layer 64 is constructed by alternately stacking a plurality of barrier layers 64a composed of GaInN and a plurality of quantum well layers 64b which differ in composition.
In a method of fabricating such a conventional semiconductor light emitting device, generally used as the sapphire substrate 61 is one having an approximate (0001) surface as its principal plane, to successively form the respective layers from the buffer layer 62 to the p-type cap layer 65 on the sapphire substrate 61. In this case, the respective layers from the n-type contact layer 63 to the p-type cap layer 65 are grown as crystals in a [0001] direction of a nitride based semiconductor.
In a crystal, having no center of symmetry, having a zinc-blende structure, a wurtzite structure, or the like, however, a piezoelectric effect may be generally generated by a strain. In the zinc-blende structure, for example, a piezoelectric effect is the greatest in a strain compressing or extending with respect to a [111] axis. In the wurtzite structure, the piezoelectric effect is the greatest in a strain compressing or extending with respect to a [0001] axis.
In the conventional semiconductor light emitting device, the light emitting layer 64 composed of GaInN has a quantum well structure having a (0001) plane as its principal plane. The lattice constant of a quantum well layer 64b composed of GaInN is larger than the lattice constant of the n-type contact layer 63 composed of n-GaN. Accordingly, a compressive strain is exerted on the quantum well layer 64b in an in-plane direction (a direction parallel to an interface) of a quantum well, and a tensile strain is exerted in a direction of confinement in the quantum well (a direction perpendicular to the interface). As a result, the piezoelectric effect generates a potential gradient in the quantum well layer 64b, so that a potential is low in the [0001] direction, while being high in a [0001] direction. An energy band in the light emitting layer 64 having a quantum well structure in this case is illustrated in FIG. 43. FIG. 43 illustrates five barrier layers 64a and four quantum well layers 64b. 
As shown in FIG. 43, the potential gradient exists in the quantum well layer 64b in the light emitting layer 64. Accordingly, electrons and holes which are injected as current are spatially separated from each other, as shown in FIG. 44. As a result, in the semiconductor light emitting device, luminous efficiency is reduced. Particularly in the semiconductor laser device, threshold current is raised.
When impurities are added to the quantum well layer 64b in the light emitting layer 64, the effect of decreasing the potential gradient by the movement of carriers is produced. When both p-type impurities and n-type impurities are added to the quantum well layer 64b, however, the carriers are compensated for, so that the carrier concentration is lowered. Consequently, the effect of decreasing the potential gradient by the movement of the carriers is reduced. Particularly when the respective concentrations of the p-type impurities and the n-type impurities which are added to the quantum well layer 64b are approximately equal to each other, the effect of decreasing the potential gradient by the movement of the carriers is further lowered.
Such a phenomenon also occurs in a III-V group compound semiconductor (for example, a GaInP based semiconductor, a GaAs based semiconductor, or an InP based semiconductor) other than the zinc-blende structure and the wurtzite structure, a II-VI group semiconductor, and an I-VII group semiconductor. Particularly in the nitride based semiconductor, the piezoelectric effect is great. Accordingly, the potential gradient generated by the piezoelectric effect is increased, so that the drop in luminous efficiency and the rises in threshold current and operating current become significant.
An object of the present invention is to provide a light emitting device which is high in luminous efficiency and is low in operating current or threshold current.
A light emitting device according to an aspect of the present invention comprises a first n-type layer; a first p-type layer; a light emitting layer arranged so as to be interposed between the first n-type layer and the first p-type layer and having a strain generating a piezoelectric effect; and a second n-type layer provided between at least the light emitting layer and the first p-type layer and having a wider bandgap than that of the light emitting layer, the potential in the light emitting layer whose gradient is generated by the piezoelectric effect being higher on the side of the first n-type layer than that on the side of the first p-type layer.
In the light emitting device, the second n-type layer is formed between the light emitting layer and the first p-type layer, so that electrons are moved in a direction perpendicular to an interface of the light emitting layer, the electrons and ionized donor levels are spatially separated from each other, and the potential gradient generated by the piezoelectric effect in the direction perpendicular to the interface is decreased. Consequently, the electrons and holes which are injected as current are prevented from being separated from each other. Accordingly, gain is easily obtained, thereby preventing luminous efficiency from being reduced and preventing operating current or threshold current from being raised.
The first p-type layer may comprise a first cladding layer, and the bandgap of the second n-type layer may be narrower than that of the first cladding layer. In this case, the refractive index of the second n-type layer is larger than that of the first cladding layer, so that the second n-type layer functions as an optical guide layer.
A light emitting device according to another aspect of the present invention comprises a first n-type layer; a first p-type layer; a light emitting layer arranged so as to be interposed between the first n-type layer and the first p-type layer and having a strain generating a piezoelectric effect; and a second p-type layer provided between at least the light emitting layer and the first n-type layer and having a wider bandgap than that of the light emitting layer, the potential in the light emitting layer whose gradient is generated by the piezoelectric effect being higher on the side of the first n-type layer than that on the side of the first p-type layer.
In the light emitting device, the second p-type layer is formed between the light emitting layer and the first n-type layer, so that holes are moved in the direction perpendicular to the interface of the light emitting layer, the hole and ionized acceptor levels are spatially separated from each other, and the potential gradient generated by the piezoelectric effect in the direction perpendicular to the interface is decreased. Consequently, the electrons and the holes which are injected as current are prevented from being separated from each other. Accordingly, gain is easily obtained, thereby preventing luminous efficiency from being lowered and preventing operating current or threshold current from being raised.
The first n-type layer may comprise a second cladding layer, and the bandgap of the second p-type layer may be narrower than that of the second cladding layer. In this case, the refractive index of the second p-type layer is larger than that of the second cladding layer, so that the second p-type layer functions as an optical guide layer.
A material composing the light emitting layer may have a wurtzite structure. In a crystal having the wurtzite structure, the piezoelectric effect exists by the strain. Consequently, the second n-type layer is provided between the light emitting layer and the first p-type layer, or the second p-type layer is provided between the light emitting layer and the first n-type layer, so that the potential gradient in the light emitting layer generated by the piezoelectric effect is decreased.
A principal plane of the light emitting layer may be approximately perpendicular to a  less than 0001 greater than  direction. In the crystal having the wurtzite structure, the piezoelectric effect produced by the strain compressing or extending with respect to a  less than 0001 greater than  axis is the greatest. Accordingly, the effect of decreasing the potential gradient by forming the second n-type layer or the second p-type layer is significantly produced.
A material composing the light emitting layer may have a zinc-blende structure. In a crystal having the zinc-blende structure, the piezoelectric effect exists by the strain. Consequently, the second n-type layer is provided between the light emitting layer and the first p-type layer, or the second p-type layer is provided between the light emitting layer and the first n-type layer, so that the potential gradient in the light emitting layer generated by the piezoelectric effect is decreased.
A principal plane of the light emitting layer may be approximately perpendicular to a  less than 111 greater than  direction. In the crystal having the zinc-blende structure, the piezoelectric effect by the strain compressing or extending with respect to a  less than 111 greater than  axis is the greatest. Accordingly, the effect of decreasing the potential gradient by forming the second n-type layer or the second p-type layer is significantly produced.
The strain generating a piezoelectric effect may include a strain for compressing the light emitting layer in an in-plane direction of the light emitting layer. In this case, the piezoelectric effect exists by the strain for compressing the light emitting layer in the in-plane direction of the light emitting layer. Consequently, the second n-type layer is formed between the light emitting layer and the first p-type layer or the second p-type layer is formed between the light emitting layer and the first n-type layer, so that the potential gradient generated by the piezoelectric effect is decreased.
The strain generating a piezoelectric effect may include a strain for extending the light emitting layer in an in-plane direction of the light emitting layer. In this case, the piezoelectric effect exists by the strain for extending the light emitting layer in the in-plane direction of the light emitting layer. Consequently, the second n-type layer is formed between the light emitting layer and the first p-type layer or the second p-type layer is formed between the light emitting layer and the first n-type layer, so that the potential gradient generated by the piezoelectric effect is decreased.
A material composing the light emitting layer may be a III-V group compound semiconductor. The III-V group compound semiconductor may be a nitride based semiconductor including at least one of boron, gallium, aluminum, and indium. Particularly in the nitride based semiconductor, the piezoelectric effect is great, so that the potential gradient generated by the piezoelectric effect is increased. Consequently, the effect of decreasing the potential gradient by forming the second n-type layer between the light emitting layer and the first p-type layer or forming the second p-type layer between the light emitting layer and the first n-type layer is significantly produced.
A material composing the light emitting layer may be a II-VI group compound semiconductor or a I-VII group compound semiconductor. Also in this case, the potential gradient generated by the piezoelectric effect can be decreased by forming the second n-type layer between the light emitting layer and the first p-type layer or forming the second p-type layer between the light emitting layer and the first n-type layer.
In the light emitting device, the light emitting layer may have a quantum well structure comprising one or more well layers having a strain generating a piezoelectric effect and two or more barrier layers arranged so as to interpose the well layer therebetween, and the potential in the well layer whose gradient is generated by the piezoelectric effect may be higher on the side of the first n-type layer than that on the side of the first p-type layer. In this case, the potential gradient generated by the piezoelectric effect in the direction of confinement in the quantum well structure is decreased by forming the second n-type layer between the light emitting layer and the first p-type layer or forming the second p-type layer between the light emitting layer and the first n-type layer.
Acceptor levels and/or donor levels may be nonuniformly formed in the light emitting layer having the quantum well structure in order to decrease the potential gradient generated by the piezoelectric effect in the direction of confinement in the quantum well structure.
In this case, acceptor levels and/or donor levels are nonuniformly formed in the light emitting layer having the quantum well structure, so that the potential gradient generated by the piezoelectric effect in the direction of confinement in the quantum well structure is further decreased. Consequently, the electrons and the holes which are injected as current are further prevented from being separated from each other. Accordingly, gain is easily obtained, thereby preventing luminous efficiency from being reduced and preventing operating current or threshold current from being further raised.
The light emitting layer may have a MQW structure comprising two or more well layers and three or more barrier layers with the well layer interposed therebetween. Further, the light emitting layer may have a single quantum well (SQW) structure comprising one well layer and two barrier layers with the well layer interposed therebetween.
In the well layer, more acceptor levels may be formed in its portion on the side of a higher potential generated by the piezoelectric effect than those in its portion on the side of a lower potential.
In this case, the holes are moved in the direction of confinement in the quantum well structure, and the holes and the ionized acceptor levels are spatially separated from each other. Consequently, the potential gradient in the well layer generated by the piezoelectric effect is decreased.
In the well layer, more donor levels may be formed in its portion on the side of a lower potential generated as a result of the piezoelectric effect than those in its portion on the side of a higher potential.
In this case, the electrons are moved in the direction of confinement in the quantum well structure, and the electron and the ionized donor levels are spatially separated from each other. Consequently, the potential gradient in the well layer generated by the piezoelectric effect is further decreased.
In the barrier layer, more acceptor levels may be formed in its portion in contact with an interface of the well layer on the side of a higher potential generated as a result of the piezoelectric effect than those in its portion in contact with an interface of the well layer on the side of a lower potential.
In this case, the holes are moved in the direction of confinement in the quantum well structure, so that the holes and the ionized acceptor levels are spatially separated from each other. Consequently, the potential gradient in the well layer generated by the piezoelectric effect is further decreased.
In the barrier layer, more donor levels may be formed in its portion in contact with an interface of the well layer on the side of a lower potential generated as a result of the piezoelectric effect than those in its portion in contact with an interface of the well layer on the side of a higher potential.
In this case, the electrons are moved in the direction of confinement in the quantum well structure, so that the electrons and the ionized donor levels are spatially separated from each other. Consequently, the potential gradient in the well layer generated by the piezoelectric effect is further decreased.
Both the acceptor levels and the donor levels may be formed in the light emitting layer having the quantum well structure. In this case, the electrons and the holes are compensated for, so that few carriers are generated by forming the acceptor levels and the donor levels. However, the potential gradient generated by the piezoelectric effect is decreased by the ionized acceptor levels and the ionized donor levels.
The concentration of the acceptor levels and the concentration of the donor levels may be approximately equal to each other. In this case, the carriers are easily compensated for. However, the effect of decreasing the potential gradient is high.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.